1. Field of the Invention
The present invention relates to an electromagnetically driven transport device for electromagnetically vibrating a transport unit to transport an article to be transported, and a weighing apparatus utilizing such transport device.
2. Description of the Prior Art
The transport device of the kind referred to above is so designed and so configured that the transport unit such as, for example, a trough for the support thereon of articles to be transported is mounted on a support base through a bracket that can be vibrated forwards and rearwards by an electromagnet to thereby transport the articles successively. In such case, in order to achieve an efficient transport of the articles, the driving frequency of an electric power source for driving the electromagnet is chosen to be equal to the natural frequency of the transport device.
Methods hitherto known to coordinate the driving frequency with the natural frequency of the transport device includes the use of an inverter for variably setting the driving frequency so that the driving frequency at which the electric power supplied can be maximized is used to actually drive the electromagnet to achieve the transport such as disclosed in, for example, the Japanese Patent No. 2,770,295 (JPA3-223014); and the use of an inverter for variably setting the driving frequency so that the driving frequency at which the current can be maximized can be determined automatically at which the transport device is driven to achieve the transport of the articles such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 7-060188.
It has, however, been found that with the transport device that is driven to effect the actual transport at the driving frequency at which the electric power supplied can be maximized, a detecting circuit for detecting a voltage and a current is needed for calculation of the electric power and the transport device is accordingly complicated in structure. On the other hand, with the transport device that is driven at the driving frequency that is automatically determined at which the electric current can be maximized requires the use of a calculating unit and a storage unit of a correspondingly large capacity for detecting the electric current at all times and then to calculate the maximum value of the electric current and the transport device is therefore complicated in structure and high in cost.
The present invention has therefore been devised to substantially eliminate the above discussed problems and inconveniences and is intended to provide a simple and low-cost, electromagnetically driven transport device capable of achieving an efficient transport and, also, a weighing apparatus utilizing at least one such electromagnetically driven transport device.
In order to accomplish the foregoing object, the present invention provides an electromagnetically driven transport device which includes a transport unit for transporting an article to be transported; an electromagnetically driven vibrating unit for vibrating the transport unit to transport the article; a control unit for controlling a driving frequency of a driving power source of the vibrating unit; a time detecting unit for detecting a time length during which when the driving frequency is changed the driving current of the vibrating unit at each of frequency exceeds a reference current value; and a frequency setting unit for setting an actual driving frequency based on the time length detected by the time detecting unit.
According to the present invention, when the driving frequency of the driving power source of the vibrating unit is varied by the control unit, the time detecting unit detects the time length during which the driving current of the vibrating unit at each of the frequencies exceeds the reference current value and the frequency setting unit sets the actual driving frequency based on the time length so detected. Accordingly, with no need to use any detecting circuit for detecting a voltage or a current, any calculating unit and any storage unit of a large capacity, the transport unit can be vibrated at the driving frequency close to the natural frequency of the transport device and, therefore, it is possible to achieve an efficient transport drive with a simplified and low-cost structure.
In a preferred embodiment of the present invention, the time detecting unit may include an element that is activated when the driving current exceeds the reference current value, and is operable to detect the time length depending on an operating condition of such element. Alternatively, the time detecting unit may include a voltage comparator for comparing a voltage corresponding to the driving current with a voltage corresponding to the reference current value for outputting an output signal, so that the time length can be detected in dependence on a condition of the output signal from the voltage comparator. With this configuration, the time detecting unit can have a simple and low-cost structure to achieve an efficient transport drive.
In a preferred embodiment of the present invention, the electromagnetically driven transport device may further include a memory for storing a reference current value and a detecting means for detecting a current value of the driving current, and the time detecting unit is operable to detect the time length based on the detected current value and the reference current value.
According to this configuration, since the reference current value appropriate to a particular kind of the article to be transported and/or a particular type of the transport unit can be read out from the storage unit and be then used, it is possible to achieve an efficient transport drive appropriate to various modes of operation of the transport device.
In a preferred embodiment of the present invention, the frequency setting unit may set the actual driving frequency based on change of the time length. Accordingly, even though the time length represents various values, the actual driving frequency appropriate to such change of the time length can be set.
In a still further preferred embodiment of the present invention, detection performed by the time detecting unit is carried out during a trial run of the transport device and drive of the transport device at the actual driving frequency so set is carried out during an actual run of the transport device. In such case, from the very beginning of the actual run, the transport of the articles can be efficiently performed with the transport device driven at the proper actual driving frequency.
The present invention also provides a weighing apparatus which includes a plurality of electromagnetically driven transport devices each being of the structure as discussed above and a weighing unit for weighing an article transported by each of the electromagnetically driven transport device. In this weighing apparatus, the actual driving frequency for each of the electromagnetically driven transport devices is preferably individually set by the frequency setting unit.
With this weighing apparatus, since for each of the transport devices employed therein the transport unit can be vibrated at the actual driving frequency about equal to the natural frequency of the transport device, it is possible to achieve an efficient transport drive.